1. Field of the Invention
This invention relates to a process and apparatus for cyclonic combustion of fossil fuels, in particular, natural gas, which provides low pollutant emissions as well as high system efficiencies. The process and apparatus of this invention are particularly suited to firetube boilers.
2. Description of the Prior Art
Conventional combustion of fossil fuels in air produces elevated temperatures which promote complex chemical reactions between oxygen and nitrogen in the air, forming various oxides of nitrogen as by-products of the combustion process. These oxides, containing nitrogen in different oxidation states, generally are grouped together under the single designation of NO.sub.x. Concern over the role of NO.sub.x and other combustion by-products, such as sulphur dioxide and carbon monoxide, in "acid rain" and other environmental problems is generating considerable interest in reducing the formation of these environmentally harmful by-products of combustion.
In addition to NO.sub.x and carbon monoxide (CO) emissions of total hydrocarbons (THC) and carbon dioxide (CO.sub.2) are also of considerable concern. Natural gas is a low emission, high efficiency fuel which can help reduce these emissions. As a result, numerous ultra-low emission, natural gas fired combustion systems are under development.
One of the advanced methods to achieve ultra-low emissions is cyclonic combustion in which a swirl is imparted to both the combustion air and natural gas as they are injected into the combustion chamber, resulting in strong internal combustion products recirculation--both tangential and axial. This inherent recirculation characteristic has been effectively exploited in burner/combustor designs to achieve ultra-low emissions of NO.sub.x, CO and THC, high combustion intensity and combustion density, very high combustion efficiency, and high heat transfer to the cooled walls, even at relatively low flame temperatures.
Swirl, or a cyclonic flow pattern, can be imparted to the combustion air and natural gas in several known ways, most notably the use of mechanical swirlers disposed in the nozzle through which the combustion air and/or natural gas are injected into the combustion chamber or the use of tangential injection means for tangentially injecting the combustion air and/or natural gas into the combustion chamber.
There are two major cyclonic combustor designs, adiabatic combustors which, although known to provide high specific heat release, are known to produce high combustion temperatures, and thus high NO.sub.x emissions at low excess air operation, and non-adiabatic combustors, that is, combustors with cooled walls.
U.S. Pat. No. 4,920,925 teaches a boiler having a cyclonic combustor comprising a substantially cylindrical, uncooled and refractory lined primary combustion chamber, a substantially cylindrical secondary combustion chamber in fluid communication with and substantially longitudinally aligned with the downstream end of the primary combustion chamber, means for supplying air and fuel directly into the primary combustion chamber in a manner which forms a cyclonic flow pattern of gases within the primary combustion chamber and the secondary combustion chamber, and a substantially cylindrical exit throat at the downstream end of the secondary combustion chamber aligned substantially concentrically with the secondary combustion chamber for exhausting hot gases from the secondary combustion chamber. The walls of the secondary combustion chamber are cooled. See also U.S. Pat. No. 4,879,959, U.S. Pat. No. 5,029,557, U.S. Pat. No. 4,860,695, and U.S. Pat. No. 4,989,549 which generally teach different types of swirl or cyclonic combustors. See also U.S. Pat. Nos. 3,974,021 and 3,885,906 which teach a process and apparatus for thermal treatment of industrial waste water using cyclonic combustion of fuel in which the walls of the top portion of the combustion chamber are provided with an insulating lining while the walls of the lower portion of the combustor below the level of a burner apparatus are provided with a chilled lining having a circulatory or evaporative water cooling system.
U.S. Pat. No. 3,934,555 discloses a cast iron modular boiler having a cylindrical combustion chamber into which a mixture of gaseous fuel and air is introduced parallel to its longitudinal axis in a manner which imparts a rotational flow around the longitudinal axis. The combustion gases are recirculated internally, thereby causing dilution of gases in the boiler. The combustion chamber is encircled by a water circulation conduit and cooled by a stream of cold water that circulates through the conduit. Heat is removed from the combustion chamber as hot water.
U.S. Pat. No. 4,007,001 teaches a combustion process producing low NO.sub.x emissions by tangentially introducing 0-65% of the total air required for combustion to a primary combustion zone and about 5-25% of the total air required for combustion to a secondary combustion zone where there is an orifice disposed between the primary and secondary combustion zones.
U.S. Pat. No. 3,859,786 teaches a vortex flow combustor having a restricted exit from the combustion chamber.
U.S. Pat. No. 4,021,188 and U.S. Pat. No. 3,837,788 both teach staged combustion with less than the stoichiometric amount of air in the primary combustion chamber with additional air being added to the secondary combustion chamber for completion of combustion.
U.S. Pat. No. 4,575,332 teaches staged combustion in a swirl combustor with forced annular recycle of flue gases to the upstream end of the primary combustion zone.
U.S. Pat. No. 4,395,223 discloses staged combustion with excess air introduced into the primary combustion zone with additional fuel being introduced into the secondary combustion zone.
U.S. Pat. No. 3,741,166 discloses a blue flame burner with recycle of combustion products with low excess air to produce low NO.sub.x while U.S. Pat. No. 4,297,093 discloses a single combustion chamber with a specific flow pattern of fuel and combustion air forming fuel-rich primary zones and fuel-lean secondary zones in the combustion chamber.